When to use a semicolon before a conjunction
 
When a sentence is made up of two simple sentences joined by a conjunction (e.g., and, or, but), it is possible to precede the conjunction with a semicolon if either of the simple sentences contains a comma(s). This is quite an outdated practice, but it still can be used to improve readability.

For example: Mark, 17, joined the Army; and Paul, Mark's younger brother, joined the Marines.
 

Semicolon before And

As covered in the lesson Commas before Conjunctions, when a conjunction (e.g., and, but, or) merges two simple sentences into one, it should be preceded by a comma. However, if one (or both) of the sentences already contains a comma (or commas), then a semicolon can be used instead of a comma to outrank the commas within the simple sentences.

Using a semicolon in this way is quite an outdated practice. However, you can still use a semicolon if you think it makes things clearer for your readers than using a comma (which would the usual practice these days).

Examples:

  • In the '60s, there were dozens of buzzards along the 7-mile trek; but, due to the decline in vermin, only 2 adults live in the area at present.
  • (This sentence is made up of two simple sentences: "In the '60s, there were dozens of buzzards along the 7-mile trek." + "Due to the decline in vermin, only 2 adults live in the area at present."
    As these two sentences both contain commas, they can merged into one sentence using a conjunction (in this case but) preceded by a semicolon.
    (Remember, it would be far more common to use a comma and not a semicolon.)

  • Mark, Dawn and Sally adore boiled spare ribs; but Julia, a staunch vegetarian,leaves the room when they are on the menu.
  • (Semicolon before but – a comma can also be used and is more common.)
 
COMPOUND SENTENCE? 

A sentence made up of two simple sentences is called a compound sentence. The conjunction that joins the two halves of a compound sentence should be preceded by a comma or possibly a semicolon.
 

See also:

What are conjunctions?
Conjunctions and commas
Using semicolons before conjunctions (and, or, but, etc.)
Using semicolons before transitional phrases (e.g. however)
Using semicolons in lists
Using semicolons to extend a sentence
List of easily confused words